The present invention pertains generally to the operation of a controller. More particularly, the present invention pertains to a method and apparatus for operating a controller to provide the controller with input protection.
Microcontrollers and microprocessors are presently in wide use. They are used to control many systems, such as heating systems. Part of the function of a controller in controlling a heating system is to monitor input signals which represent system conditions in the heating system. Based upon the signals monitored, the controller commands various output signals which, in turn, alter control of the heating system. For example, in some heating systems pressure switches are monitored, the flame in the heating system is verified, and other safety conditions are also monitored. Based upon these conditions, the controller may shut down the heating system or operate it in various modes as the conditions require.
In any case, it is generally necessary to provide input terminals or input ports to the controller with protection against transients and voltage spikes caused by, among other things, static discharge. This protection generally takes the form of a diode and resistor network which is implemented with discrete components externally to the controller integrated circuit (IC) chip. In addition, some microcontroller and microprocessor IC chips also have internal protection which takes the form of a similar diode and resistor network.
There are several reasons why these protection techniques are either ineffective or inadequate. First, present applications of microcontrollers and microprocessors require them to be very small in size. This requires chip manufacturers to continually downsize the die of microcontroller and microprocessor IC chips. In downsizing the die, the chip manufacturers are required to reduce the size of the resistors and diodes used for input protection on the die. Similarly, the external diodes and resistors used for protection are also required to be very small. Reducing the size of the input protection resistors and diodes results in a corresponding decrease in effectiveness as input protection.
Second, the input ports on the microcontroller or microprocessor are often coupled to a device, such as a thermostat, which is physically set or controlled by an operator. When the operator touches the control, a transient in the range of approximately 25 kilovolts, caused by static discharge, can be applied to the input of the microcontroller or microprocessor. However, typical resistors used in the internal and external protection techniques described above generally break down when 300-400 volts are applied across them. Hence, the internal and external protection techniques are inadequate to handle a transient in the range of 25 kilovolts.
For these reasons, there is a continuing need to provide adequate and effective techniques for protecting microcontrollers and microprocessors and other circuitry against transients.